NEHAL SALVI AND ARUNA K
Abstract
A soil bacterium capable of utilizing and degrading methyl parathion (MP), a toxic organophosphorus pesticide, as carbon source was isolated from the garden soil. The strain was identified as Bacillus thuringiensis INRS7 based on the morphological, cultural, biochemical tests and 16S rRNA sequence analysis. Primary screening was done spectrophotometrically by measuring change in absorbance of the culture at 410 nm due to formation of para- nitrophenol (PNP), a degradation product of MP. Maximum MP degrading activity by the selected isolate was obtained in optimized mineral salts medium (K2SO4, 0.015%; MgSO4.7H2O, 0.025%; CaSO4, 0.0045%; FeSO4.7H2O, 0.00015%) containing MP (187.5mg/L) with NaCl (0.1%) and tryptone (0.25%) at pH 6. The optimum activity was observed in a shaker culture (120 rpm) with the optical density of 0.5 OD530 nm, at 300C for 5 days. No significant effect on growth or the degradation of MP was seen when other phosphorous sources like KH2PO4 and K2HPO4 (0.1%) were provided to the isolate, suggesting that MP is used effectively as phosphorous source. MP was the sole source of carbon in the minimal medium used for growth by the isolate. There was a decrease in degradation of MP, when other additional carbon sources were provided; suggesting that usage of MP was under a negative feedback control. Biodegradation analysis of MP was done by HPLC which showed the presence of PNP. Thus the isolate Bacillus thuringiensis INRS7 could be considered as a potential candidate to reduce contamination by methyl parathion and hence useful in bioremediation.